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Knowledge of the interaction of sunlight with the skin is fundamental to understanding the pathogenesis, diagnosis, and treatment of more than 100 cutaneous disorders. Whenever ultraviolet (UV) or visible radiation is used to diagnose or treat a skin condition, important principles of photophysics involving absorption and emission of light underlie the success of the therapy. Sunscreen recommendations rely on an understanding of solar UV radiation and the ways in which the causative wavelengths can be minimized. Skin cancer is an epidemic clinical problem, whose pathophysiology necessitates comprehension of the photophysical, photochemical, and photobiologic events described in this chapter.
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Almost every ancient civilization worshipped a god of the sun whose healing powers were believed to be broad reaching. Even today, sun exposure is widely felt to induce a sense of well-being. In addition, sunlight is important for the synthesis of vitamin D3 and the setting of internal clocks. On the negative side, sunlight causes deleterious acute and chronic inflammatory skin reactions, skin cancer, and photoaging, and can elicit adverse reactions to certain drugs (see Chapters 91, 92, 109, 112). Although the sun is a major source of the UV and visible radiation that interacts with human skin, UV and/or visible radiation are also emitted from common sources such as fluorescent lights, incandescent bulbs, photocopy machines, and phototherapy lamps. Tanning salons are another familiar example. Thus, UV and visible radiation are a constant part of the human environment and play a role in health, disease, and therapy. Photodermatology is the study of this ...